How can you develop skill at sight-reading music?
Sight-reading music is a complicated process, as it involves recognizing and applying information presented in the written score in order to produce the indicated tones through correctly playing an instrument or using of the human voice while integrating rhythm and volume. “The best sight-readers combined strong working memories with tens of thousands of hours of practice.”
Developing your working memory could involve drills with reading text or playing music separately, with the goal of increasing the speed with which your eyes can recognize and your brain can process increasing amounts of information. You need to train your eyes to look further along the line of music, noting the information recorded there, and be able to store that information in short-term memory until you have played or sung it. Speed-reading drills may be very beneficial to this process, as they are designed to help develop skills at recognizing and retaining information while also training your eyes to move more quickly along the printed page.
Use a significant part of your practice time to become proficient at using your instrument or voice to produce common patterns of music. Playing scales and chord progressions needs to become automatic, so that you can immediately recognize and reproduce those combinations of notes when you see them while sightreading. Make sure you practice the technical skills needed for your instrument - fingering keys, adjusting bow positions, use of vocal chords. Become so familiar with time signatures or rhythm descriptions that you can approximate the tempos without reference to a metronome or watch, and can keep the beat or change to a different tempo smoothly. If you can advance to being able to recognize and easily transition from one key to another, that might be another helpful skill to develop.